SPI is a common communication protocol used by many different devices. For example, SD Card modules, RFID card reader module, and 2.4GHz Transmitters/Receivers all use SPI to communicate with microcontrollers. Devices communicating via SPI are in a master-slave relationship. The master is the controlling device (usually a microcontroller), while the slave (usually a sensor, display, or memory chip) takes instruction from the master. The simplest configuration of SPI is a single master, single slave system, but one master can control more than one slave.The signal level model implementation of the SPI protocol is as follows:

* Master and Slave module is connected by two in-out ports for bidirectional data transfer.

* Ports are MOSI(Master Out Slave In), MISO(Master In Slave Out).

* Clock is generated by the master module.

* Data from the buffer of 1-byte size loaded into master and slave shift registers.

* Shift register of master and slave starts shifting by 1 bit through MOSI or MISO line at every clock cycle for bidirectional mode.

* At the end of 8-bit transmission data in the shift register are gets exchanged as the LSB of the master shift register gets transferred to MSB of the slave shift register and LSB of the shift register are getting transferred to the MSB of the master shift register due to shifting.